This invention relates to a blind rivet and, more particularly, to a closed-end blind rivet with a crimped shank and method of manufacture thereof.
Blind rivets are well-known and generally comprise a mandrel having a pulling stem with a radially enlarged head attached at one end and a tubular shell having a generally cylindrical shaped shank having a radial flange formed at one end of the shank for engaging a face of the workpiece. An axial bore is formed through the flange and passes a substantial way into the shank. The radially enlarged head of the mandrel is located within the shank towards the end remote from the flange adjacent an end of the bore. The enlarged head has a circumference which is greater than that of the bore. The pulling stem extends from the enlarged head through the length of the bore and extends away from the body of the rivet. A breakneck is formed on part of the stem and which is located within the shank. When the rivet is set, the flange is held stationary whilst the exposed part of the pulling stem is pulled axially away from the flange, the enlarged head being forced to pass through at least part of the bore. Because the diameter of the pulling head is larger than the bore, it causes the shank to collapse, thus forming an annular bulge or fold which projects radially outwardly. The radial outward bulge forms a blind head and engages the blind side surface of the workpiece which is the opposite side of workpiece to that of the flange. Once the pulling force on the stem exceeds a predetermined amount, the breakneck breaks leaving the rivet set. The rest of the stem can then be removed and discarded.
One type of blind rivet comprises a shank having circumferential grooves spaced at intervals along the length of the shank. When such rivets are set, the grooves collapse in an axial direction, the sections of the rivet body between the grooves expanding radially to form the characteristic “cottage loaf” setting. The depth of the grooves is critical in certain extreme conditions for example when the rivet is set in an oversized hole whilst needing to provide an air and water tight fixing. This is particularly difficult when the rivet bodies are made of intractable material such as stainless steel, carbon steel or the like. If the grooves are too deep the rivet body will crack and if they are too shallow, the grooves will not provide sufficient resistance to prevent the mandrel head from pulling into or even through the rivet body.
This can result in two problems. Firstly, during setting in softer materials, the grooves are required to be deeper to prevent the mandrel head from pulling through the rivet body. However, this can cause fracture of the body. If the grooves are not at an optimum depth, the mandrel pulls through the body causing radial expansion of the rivet body between the grooves which can result in the splitting of the workpiece material if the expansion takes place within the bore formed in the workpiece, in which the rivet is located.
Secondly, if the grooves are less than optimum, during the setting of a rivet, the mandrel head can pull through the rivet body since there is insufficient resistance from the soft workpiece materials to help provide resistance to the head of the mandrel pulling into the rivet body. This can result in part of the stem attached to the head of the mandrel protruding from the flange giving a potentially hazardous condition.
The grooves or recesses can be formed by rolling, embossing or crimping. While the prior art patents have favored rolling or embossing, the present invention uses crimping to form the recesses or radial impressions that result in a positive leakproof setting of the blind rivet that is produced over an increased range of manufacturing tolerances than was possible in the prior art.